1. Field of Invention
This invention relates to a television camera using a solid state image sensor such as a CCD (charge Coupled Device), and particularly is directed to an iris control system for a solid state television camera.
2. Description of the Prior Art
Recently, solid state imaging devices have been developed in many laboratories due to the advancement of semiconductor technology. Among various types of solid state image sensors, a CCD imager is of particular interest.
There are two types of CCD imagers which depend on the arrangement of CCD. One is the so called frame-transfer type and the other is the interline-transfer type. The former type of CCD imager has a image sensing array having a plurality of individual light sensing units arranged in both horizontal and vertical rows, and temporary storage devices of the same number as that of the light sensing units for storing and transferring the charges accumulated in said light sensing units, and a shift register for receiving the charges sequentially from the storage devices and generating a signal representing the image. In this type of CCD imager, as the light sensing units are irradiated by the light of the image during the transfer period of the charge from the image sensing array to the temporary storage area, an undesirable charge is mixed with the transferred charge representing the image. Because of the undesirable charge, the output signal of the CCD imager includes, a so called smear signal which may be one of the causes that the black portion of the image is whitish, particularly when the image has high contrast, such as in outside filming. The interline-transfer type CCD imager also generates a smear signal although the cause of generation is different from that of the frame transfer type CCD imager due to the difference of the arrangement of CCD. The appearance of smear is inevitable in the solid state imaging devices, and it causes deterioration of the generated signal.
Generally, a television camera has on iris means in an optical system arranged in front of the light sensing device for controlling the amount of light irradiated to the light sensing device so that the output signal has the proper level. Recently, many television cameras have adopted an automatic iris control for controlling the operation of the iris means by the average level of the output signal of the light sensing device. According to said existing iris control system, if the image picked up by the television camera has a small bright spot in the dark back-ground, the average level of the output signal is regarded to be relatively small, therefore the iris means is controlled so that more light will be irradiated on the light sensing device. This causes said bright spot to be more brilliant, and then the undesirable smear signal due to the bright spot becomes larger and the reproduced image from the picked up signal becomes noisy. Further, the averaging circuit requires very complicated circuit construction, because the picked up signal is a digital signal and the field or frame memory is required for averaging the whole signal during one field or frame. Finally, a solid state light sensing device, such as a CCD imager has a limitation for the capacity for handling or transferring the charge, so a charge above the predetermined level is thrown away to, for example, an over-flow-drain constructed in a CCD chip. In other words, the output signal from the CCD imager is clipped to a predetermined over-flow-level if the light from the image becomes very bright. Therefore, the picked up signal does not always represent the exact amount of light and the iris control using an average signal may not represent the average light amount from the image and results in a lack of fidelity of control. Since the smear signal is generated proportional to the radiated light, the prior art iris control does not reduce the level of the smear signal.